Remark 1 A tensor of even order
\(\mathcal {A}\in \mathbb {R}^{I_1\times \cdots \times I_N\times J_1\times \cdots \times J_N}\)  is said to be
square if the first set of dimensions equals the second set, i.e., if \(I_k=J_k\)  for
\(k=1,2,\ldots ,N\) ; see, e.g., [1]}. The adjacency tensor of a multilayer network is of
even order and square, however, for some computations tensors of different orders are
required. This is possible when using the Einstein product by choosing the number of
dimensions over which we carry out the summation.

